1. Field of the Invention
The invention relates to oil-filled apparatus, such as transformers and to means to exerting pressures therein.
2. The Prior Art
To avoid deformation of the windings in power transformers as a consequence of the mechanical stresses during short-circuits, the windings are clamped axially against the core yoke. This is accomplished by building a permanent self-stress into the winding-core system or by means of mechanical elements which may be adjusted afterwards.
A radial de-stressing of the inner winding against the core would be the best protection against deformation of the winding during short-circuit periods, but unfortunately it is very difficult to achieve this.
Resilient elements in the axial or radial power flux would maintain part of the self-stresses. Metallic elements are cumbersome from the structural point of view and are often impossible to use for technical reasons having to do with the insulation. Elements which are based on elastic deformation of, for example, glass-fibre-reinforced plastics become yielding at high temperature. Furthermore, metallic as well as glass-fibre-reinforced elements must necessarily have a certain overall height, which makes it difficult to find a place for them. Also deformed rubber is out of the question, since even the best qualities undergo deformation and de-stressing at high temperature.
However, rubber and similar materials can be used in other ways than through deformation in order to obtain elastic elements without plastic destressing. Certain qualities of rubber are known to absorb transformer oil in their joints, thus causing it to swell. The swelling is dependent on the temperature so that it increases at increasing temperatures, but it is also reversible, which means that if the temperature decreases part of the absorbed oil drains off. Hot-vulcanized silicone rubber undergoes a pronounced swelling in transformer oil. The type of rubber and the number of cross-links per rubber molecule determine the swelling. If the correct material is used, the swelling is not destructive to the rubber.
If a swelling body is enclosed in a limited space and allowed to swell, it builds up a self-stress until equilibrium has been attained. This is characterised by the fact that the chemical potential of the oil in the rubber is the same as the chemical potential of the oil in the free liquid.